Oxide films are used in a wide variety of silicon semiconductor products. For example, oxide layers commonly act as a sacrificial layer for releasing microstructure in MEMS devices. The art thus uses a number of different processes to remove or etch oxide layers. One process uses aqueous HF to remove sacrificial oxide under MEMS sensors. Unless preventive measures are taken, however, liquid surface tension can cause the MEMS microstructures to stick together (“stiction”) upon removal from aqueous baths.
Another wafer fabrication application etches oxides by exposing them to HF vapor (hydrofluoric acid vapor). Such a vapor process is attractive because it substantially eliminates the surface tension that causes stiction. For a number of reasons, it often is preferable to apply the HF vapor with portions of the wafer protected by an organic mask. More specifically, among other reasons, an organic mask may be more readily removable from an underlying surface.
Undesirably, however, prior art organic masks known to the inventors generally cannot sufficiently block HF vapor when used in these applications. Specifically, the HF vapor often does not attack/degrade the organic material itself. Rather, it is an ineffective barrier—the HF vapor often diffuses through the organic material. In that case, after diffusing through the organic mask, the HF vapor may attack/degrade the underlying material (e.g., the wafer), causing the mask to debond from the reacting surface. Hard (i.e., inorganic) masks are sometimes an alternative, but they impose other process limitations. It should be noted that the term “etch” is used herein generally includes chemical reactions, cleaning and removal of surface films, particulates, and contaminants, as well as bulk removal of material.